1. Field of the Invention
This application relates to a motor rotor variable frequency speed control system, and in particular, to a system for realizing rotor variable frequency speed control asynchronously and simultaneously by driving multiple motors via one inverter.
2. Description of the Related Art
Motor is the prime mover in each operating mechanism of a crane, blower fan and water pump. It converts electric energy into mechanical energy. Taking crane as an example, motors can drive a crane to perform various different mechanism movements such as lifting (or falling), luffing, revolving and walking, thereby accomplishing the field operation task of the crane.
FIG. 1 is a schematic diagram showing a traditional crane to realize variable frequency speed control using motors of different jobs. It can be seen from the figure that, the system converts the constant voltage constant frequency AC power supply provided by an electric network into DC power supply via a rectifier bridge, and then converts the DC power supply, by an intermediate circuit, into AC power supply of different working frequencies via an inverter bridge, so as to drive the motor to rotate.
It is supposed that: the frequency of the electric network power supply is fo, and the working frequency of the motor is fm,
Then, fm=§fo is true,
Here, § is slip ratio.
During the field operation of a crane, it generally accomplishes four different operations, lifting, luffing, revolving and walking. Therefore, each corresponding actuating mechanism needs a different motor to provide different electric energy that will be converted to a different mechanical energy. In other words, during different operations of the crane, the motor rotating speed needed is different, that is, the working frequency fm of the motor is different. However, in the traditional motor variable frequency speed control system, one inverter bridge can only convert the working frequency of one motor and carry out variable frequency speed control on one motor, which is so called “one-driving-one” technology. Apparently, for the four different operations of the crane, four inverter circuits are required to realize “AC-DC-AC” conversion during which transduction is performed twice. Thus, the respective working frequency needed by multiple motors will be generated, so that the lifting, luffing, revolving and walking operation during the field operation of the crane will be accomplished respectively.
In conclusion, for the above traditional motor speed control system, its frequency adjustment range is broad, and it will not be limited by the frequency of the electric network; not only the forced transduction, but also the load transduction, may be employed. For such a speed control system, except that the loss of slipping function is great and the efficiency is low at low speed, most prominently, four inverters are generally used, thus the system will be bulky, massy and costly, and it will be very difficult to implement.
In recent years, with the rapid development of frequency conversion technologies, especially with the application of vector control technologies and direct torque control technology, frequency conversion technology becomes increasingly mature, and it takes the leading position in AC transmission due to its broad speed control range, high steady speed precision, rapid dynamic response and its capability of reversible operation in the four quadrants of the rectangular axis. Its speed control capability can completely match that of DC transmission, and there appears a tendency that DC transmission is being replaced by it. However, in the frequency conversion technologies employed by overseas jack mechanisms, it still uses one frequency converter for one function, and one inverter bridge is equipped for one frequency converter; for the four functions during the normal operation of a crane, four inverter bridges are still needed. If energy feedback function is to be added to the variable frequency speed control system, another four inverter bridges need to be added, which is nonpaying apparently. Therefore, for the related products of many overseas companies, “one-driving-one” mode is still employed to accomplish the normal operation of a crane, for example, the products such as YASKAWA (Japan), Siemens (Germany), ABB (Switzerland) and Schneider (France), which can be seen everywhere in related application field in China, and of which the prices are very high.
Directed to the serious faults of the above existing frequency conversion technologies, studies have been made by the inventor on how to drive four motors via one inverter so as to realize rotor variable frequency speed control via the turnon and turnoff of each chopper and make a crane to accomplish four works such as lifting, luffing, revolving and walking in real time when the inverter is positioned at its minimum inverting angle during the operation of the crane. Patent applications have been filed successively by the inventor, and three utility model patents have been granted by State Intellectual Property Office of the People's Republic of China, with the Patent Number of “ZL002324369”, “ZL01212245” and “ZL2007200870857” respectively. At the same time, two patents for invention, with the application number of “2008100941476” and “2008100482526”, have been filed this year. However, problems, such as how to provide appropriate forward and inverse output control voltage to effectively turn on and turn off each chopper, how to collect the rotor phase voltage and the rectifier output DC to rapidly establish a grid control electric field by the chopper so as to guarantee the normal and orderly work of the system, and how to make the system work reasonably according to the rated power of the motor, still need to be solved.